An on-board vehicle navigation system can be useful for assisting the driver of an automobile in navigation. An example of an on-board automobile navigation system is a system which provides the driver with a visual display of a street map of a certain area, using a stored map database, and indicates the position and movement of the vehicle as the vehicle travels. In response to the driver's selecting a destination from the database, the system computes a best route from a starting location to the destination and then provides instructions to the driver to guide the driver to the destination. The instructions are given in the form of recorded or synthesized speech.
Automobile navigation systems like the one described above may use a technique known as "dead reckoning" to estimate the position of the vehicle as the vehicle travels. In dead-reckoning, the heading of the vehicle and the distance traveled from a previously calculated position are first determined based on data received from on-board sensors (e.g., compass and odometer). A new position of the vehicle is then estimated by applying the measured parameters to well-known equations. Dead reckoning is subject to certain sources of error, however, which accumulates as the vehicle travels. Therefore, the estimated position of the vehicle is periodically compared to the street map database and adjusted to correct for such error. The system may also include a high-accuracy positioning system, such as Global Positioning System (GPS) or the like.
Certain problems are associated with automobile navigation systems in the prior art, however. For example, a driver of an automobile in which a navigation system is operating may stop and turn off the automobile before reaching the selected destination. In such cases, the navigation systems in the prior art generally determine whether to resume route guidance (i.e., whether to resume providing instructions to the driver) only after the vehicle is restarted. That is, the decision whether to resume route guidance is made in the start-up routine of the navigation system, which is invoked when the vehicle is restarted. A disadvantage associated with such prior art is that it takes time for the navigation system to recall the desired destination, recompute the best route from the current location of the vehicle, and then determine whether to resume route guidance. The time that this process takes can be annoying to the driver, since the driver may require navigation instructions before proceeding.
In addition, some on-board automobile navigation systems in the prior art generally determine whether or not to resume route guidance based upon the direct (straight line) distance from the current location of the vehicle to the desired destination. In particular, such systems might resume route guidance only if the direct distance to the destination exceeds a predetermined distance. A disadvantage of this technique is that the actual driving distance may be much larger than the direct distance to the desired location. For example, the vehicle may be located physically very close to the selected destination, yet the vehicle may have to travel a significant distance on streets to reach the destination. The result is often a faulty decision on whether to resume guidance.
Hence, what is needed is a technique for reducing the amount of time consumed in the start-up routine of an automobile navigation system, and in particular, reducing the time consumed in determining whether to resume route guidance after a power-down of the system en route. What is further needed is a technique for improving the accuracy of the determination whether to resume route guidance in such cases.